1. Field of the Invention
The present invention relates to a differential transmission circuit and a common mode choke coil.
2. Background Information
In recent years there have been increasing demands for the transmission of high-frequency signals in conjunction with the diversification of electronic devices. In general, differential transmission is used for high-frequency signal transmission.
In particular, when differential transmission is applied to the transmission of high-frequency signals, electromagnetic interference affecting external electronic devices can be reduced. Moreover, differential transmission can reduce the effects of electromagnetic interference from external electronic devices.
Consider a case in which there is strong electromagnetic interference from external electronic devices, which affects differential transmission and causes noise to be carried in two transmission signals. When the noise carried in two signals is at such a high level that it exceeds the input range of the differential receiver, there is concern that the differential receiver will be subject to malfunction due to the noise in the two signals. Generally, a common mode choke coil is connected to the input terminal of the differential receiver of the differential transmission circuit in order to eliminate the noise in the signals. A common mode choke coil has the function of eliminating noise. Consequently, noise penetration in the differential receiver is prevented.
The noise suppression capability of the common mode choke coil, that is, the common mode impedance, has frequency characteristics. Therefore, the common mode choke coil cannot have a common mode impedance that is highly uniform in all frequency bands. Since the noise suppression capability is low in the frequency ranges in which the common mode choke coil has low common mode impedance, it is difficult for the common mode choke coil to effectively eliminate noise from the signals when the signals carry noise due to the influence of electromagnetic interference from external electronic devices.
Common mode chokes generate a magnetic field proportional to the common mode current of the noise flowing to the common mode choke coil. When there is substantial noise in the common mode current, the common mode choke coil is susceptible to magnetic flux saturation. As a result, there is a reduction in the noise suppression capability of the common mode choke coil.
To resolve these problems, Japanese Laid-Open Patent Publication No. 2002-261842 discloses a method for reducing the amount of noise flowing to the common mode choke coil using terminators. FIG. 23 is a diagram of the differential transmission circuit described in Japanese Laid-Open Patent Publication No. 2002-261842.
According to this construction, noise in a first signal and a second signal is divided and allocated to first and second terminators 710 and 711, and a common mode choke coil 705. Accordingly, even when the first and second signals are in the low frequency band where the common mode choke coil 705 noise suppression capability is low, noise can be eliminated by reducing the amount of noise flowing in the common mode choke coil 705. Furthermore, the phenomenon of magnetic flux saturation in the common mode choke coil 705 can also be reduced by reducing the amount of common mode current noise flowing to the common mode choke coil 705. Accordingly, the amount of noise reaching the differential receiver 709 is also inevitably reduced even though it cannot be eliminated by the common mode choke coil 705.
Furthermore, in order to avoid a decrease in the noise suppression capability of the common mode choke coil, Japanese Laid-Open Patent Publication No. 2003-046655 discloses a method which uses terminators to reduce the amount of noise flowing to the common mode choke coil, and connects external devices and the transmission lines by means of shielded serial cables. FIG. 24 is a diagram of the differential transmission circuit of Japanese Laid-Open Patent Publication No. 2003-046655.
In this construction, the noise carried by the shield 762 and the noise flowing to the first and second terminators 760 and 761 flow to the common mode choke coil 755. Therefore, the common mode choke coil 755 generates a counter electromotive force relative to the noise from the shield 762 and first and second terminators 760 and 761, and not only relative to the remaining noise flowing to the common mode choke coil 755. Accordingly, the common mode choke coil 755 has a higher noise suppression capability than the method in Japanese Laid-Open Patent Publication No. 2002-261842. Consequently, the amount of noise reaching the differential receiver 759 is reduced even though it cannot be eliminated by the common mode choke coil 755.
However, differential transmissions by these methods present the following problems.
In Japanese Laid-Open Patent Publication No. 2002-261842, when the noise in the first and second signals is so great it cannot be eliminated using the first and second terminators 710 and 711 and the common mode choke coil 705, the noise cannot be eliminated before reaching the tolerance limit of the differential receiver 709. There is therefore concern that the noise may cause malfunction of the differential receiver 709, which may in turn damage the semiconductor device. This phenomenon must be taken into account particularly when using differential transmissions for signal transmission between devices installed in automobiles. FIG. 25 shows an example in which differential transmission is used between devices installed in a vehicle. In addition to an engine, brakes and the like, onboard devices A and B are installed in the automobile shown in FIG. 25. The onboard devices A and B are a car navigation device and audio device or the like. The onboard devices A and B are provided with differential transmission transceivers C and D, respectively. The onboard device A and onboard device B transmit and receive signals using the differential transmission transceivers C and D. The engine produces strong electromagnetic interference. Therefore, the signals transmitted between the onboard devices A and B are affected even when using differential transmission. As a result, there is a high probability that malfunction will occur due to the input noise which cannot be eliminated by the differential receiver within the differential transmission circuit. Furthermore, when differential transmission does not eliminate noise from the transmission signal, the electromagnetic field can cause interference for external devices, and there is a possibility that the braking device and the like may be affected and malfunction. Accordingly, this method is difficult to apply in environments with extremely high levels of electromagnetic interference from external devices as in the case of automobiles.
In Japanese Laid-Open Patent Publication No. 2003-046655, there is increased weight and cost because the transmission line 752 is covered by the shield 762. Particularly when the differential transmission circuit of Japanese Laid-Open Patent Publication No. 2003-046655 is installed in an automobile, the weight of the shielding is 50 to 100 kg. Accordingly, the weight and cost of the shield cannot be ignored.
The present invention solves these problems by providing a differential transmission circuit and common mode choke coil used in same, which improves protection for the differential receiver and eliminates noise by a differential transmission circuit structure that is also light-weight and inexpensive.